The disclosure herein relates generally to medical devices and, more particularly, to pacing circuitry and methods of pacing.
A variety of medical devices have been described which deliver a therapy to patients. For example, some medical devices are entirely or primarily located external to the body of a patient, while others are implantable in a patient. Some medical devices may use stimulation electrodes, sense electrodes, or other apparatus to deliver such therapy to a patient (e.g., deliver pacing or electrical stimulation to, the heart, muscle, nerve, brain, stomach, or other organs or tissue). For example, medical leads are configured to allow electrodes or sensors to be positioned at desired locations for delivery of stimulation or sensing (e.g., those having a proximal portion connected to medical device housing with a distal portion having electrodes or sensors located thereon). Other electrodes or sensors may be located on or within the medical device housing, or have a wireless connection to the system.
Cardiac pacemakers or cardioverter-defibrillators, for example provide therapeutic electrical stimulation to the heart via electrodes, e.g., electrodes carried by one or more leads. The electrical stimulation may include signals such as pulses or shocks for pacing, cardioversion or defibrillation. In some cases, a medical device may sense intrinsic depolarizations of the heart, and control delivery of stimulation signals to the heart based on the sensed depolarizations. For example, upon detection of abnormal rhythm, such as bradycardia, an appropriate electrical pacing stimulation signal or signals may be delivered to maintain or restore a normal rhythm. In other cases, for example, a medical device may deliver rapid pacing pulses to the heart of the patient upon detecting an abnormal rhythm, such as tachycardia. Still further, high voltage shocks may also be delivered for purpose of cardioversion of a tachycardia, or for defibrillation of the heart upon detecting fibrillation.
Further, medical device equipment may also be used to conduct temporary cardiac pacing. For example, such cardiac pacing may be performed at the end of an operative procedure, such as open heart surgery. External or temporary pulse generators (e.g., external pacemakers) are electronic devices packaged for control of the stimulation and to be located outside the patient's body. Examples of temporary or external pulse generators include, for example, the Medtronic Single-Chamber Model 5348 External Pulse Generator, and the Medtronic Dual-Chamber Model 5388 External Pulse Generator. For example, cardiac pacing leads may be attached to the patient's heart and be connected to an external pulse generator (EPG) for delivery of stimulation to the patient's heart. Various types of pacing leads, such as heart wires, are used to provide connection of the external pulse generator to the patient.
In one or more conventional types of pulse generators, such as external pulse generators, a constant current output is used for pacing. A constant current circuit is used that varies the output voltage to obtain a fixed output current regardless of lead impedance. One type of constant current circuit uses a current mirror transistor pair to deliver the output current. However, designing a constant current circuit using a current mirror may be challenging in that it can be difficult to match transistors to obtain equal current balancing in the circuit. Further, for example, accurately generating a low current bias signal (e.g., typically required in such circuits) in an efficient and noise immune manner is difficult. Still further, for example, many constant current designs use some sort of defibrillation protection, however, in some cases damage can still be caused by a defibrillation pulse directly applied to a patient in parallel with the device.